SOx tolerant NOx trap catalysts and methods of making and using the same

ABSTRACT

The present invention relates to sulfur tolerant catalyst composites useful for reducing contaminants in exhaust gas streams, especially gaseous streams containing sulfur oxide contaminants. More specifically, the present invention is concerned with improved NOx trap catalysts for use in diesel engines as well as lean burn gasoline engines. The sulfur tolerant NOx trap catalyst composites comprise a platinum component, a support, and a NOx sorbent component prepared by hydrothermal synthesis. The NOx sorbent component comprises a first metal oxide and a second metal oxide. The metal in the first metal oxide is selected from the group consisting of aluminum, titanium, zirconium, silicon, and composites thereof, and the metal in the second metal oxide is selected from the group consisting of Group IIA metals, Group II metals, Group IV metals, rare earth metals, and transition metals. The metal in the first metal oxide is different from the metal in the second metal oxide. The sulfur tolerant NOx trap catalyst composites are highly effective with a sulfur containing fuel by trapping sulfur oxide contaminants which tend to poison conventional NOx trap catalysts. The sulfur tolerant NOx trap catalyst composites are particularly suitable for diesel engines because the composites can be regenerated at moderate temperatures with rich pulses, rather than at high temperatures.

This application is a continuation-in-part of U.S. patent applicationSer. No. 09/771,280, filed Jan. 26, 2001 now U.S. Pat. No. 6,585,945 B2.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to sulfur tolerant catalyst compositesuseful for reducing contaminants in exhaust gas streams, especiallygaseous streams containing sulfur oxide contaminants. More specifically,the present invention is concerned with improved NOx trap catalysts foruse in diesel engines as well as lean burn gasoline engines. The sulfurtolerant NOx trap catalyst composites comprise a platinum component, asupport, and a NOx sorbent component prepared by hydrothermal synthesis.The NOx sorbent component comprises a first metal oxide and a secondmetal oxide. The metal in the first metal oxide is selected from thegroup consisting of aluminum, titanium, zirconium, silicon, andcomposites thereof, and the metal in the second metal oxide is selectedfrom the group consisting of Group IIA metals, Group III metals, GroupIV metals, rare earth metals, and transition metals. The metal in thefirst metal oxide is different from the metal in the second metal oxide.The sulfur tolerant NOx trap catalyst composites are highly effectivewith a sulfur containing fuel by trapping sulfur oxide contaminantswhich tend to poison conventional NOx trap catalysts. The sulfurtolerant NOx trap catalyst composites are particularly suitable fordiesel engines because the composites can be regenerated at moderatetemperatures with rich pulses, rather than at high temperatures.

2. Related Art

Diesel powered vehicles represent a significant portion of the vehiclemarket worldwide. In Europe, the market share of diesel passenger carsis about one third and is expected to grow even higher in the yearsahead. Compared to gasoline powered vehicles, diesel vehicles offerbetter fuel economy and engine durability. As diesel passenger carsbecome more popular both in Europe and elsewhere, emissions reduction isan increasingly urgent issue. In fact, Euro Stage IV regulations (year2005) are calling for a 50% reduction of NOx emissions (0.25 g/km)compared to the Stage III (year 2000) level (0.5 g/km). For somevehicles, it would be difficult to meet the Euro IV NOx emissions targetby engine improvement alone. It may be impossible to meet Euro V NOxregulations (0.125 g/km) without highly efficient after-treatmenttechnologies.

Reducing NOx from diesel exhaust is very challenging. The 3-way catalysttechnology, which is widely used in the gasoline cars, is notoperational in diesel vehicles. A 3-way catalyst requires the exhaustemissions near a stoichiometric point, neither fuel rich (reducing) norlean (oxidizing), while diesel emissions are always lean. In the early90's, the concept of NOx trap catalyst was explored for gasoline partiallean burn (PLB) engines where the NOx catalyst would trap NOx in a leanenvironment and reduce it in a rich environment.

To apply the NOx trap concept to diesel passenger cars, some specialissues related to diesel emission characteristics needed to beaddressed. The exhaust temperature for light-duty diesel vehicles istypically in the range of 100-400° C., which is much lower than thegasoline exhaust. Therefore, low temperature oxidation activity andreduction during a rich spike is critical. One of the most difficultchallenges in applying this concept is the issue of sulfur poisoning.The exhaust sulfur forms a very strong sulfate on any basic metal site,which prevents the formation of nitrate, rendering the catalystineffective for trapping NOx. As with other catalytic converters,thermal stability is another important issue for practical application.

The operation of a NOx trap catalyst is a collection of a series ofelementary steps, and these steps are depicted below in Equations 1-5.In general, a NOx trap catalyst should exhibit both oxidation andreduction functions. In an oxidizing environment, NO is oxidized to NO₂(Equation 1), which is an important step for NOx storage. At lowtemperatures, this reaction is typically catalyzed by precious metals,e.g., Pt. The oxidation process does not stop here. Further oxidation ofNO₂ to nitrate, with incorporation of an atomic oxygen, is also acatalyzed reaction (Equation 2). There is little nitrate formation inabsence of precious metal even when NO₂ is used as the NOx source. Theprecious metal has the dual functions of oxidation and reduction. Forits reduction role, Pt first catalyzes the release of NOx uponintroduction of a reductant, e.g., CO (carbon monoxide) or HC(hydrocarbon) (Equation 3). This may recover some NOx storage sites butdoes not contribute to any reduction of NOx species. The released NOx isthen further reduced to gaseous N₂ in a rich environment (Equations 4and 5). NOx release can be induced by fuel injection even in a netoxidizing environment. However, the efficient reduction of released NOxby CO requires rich conditions. A temperature surge can also trigger NOxrelease because metal nitrate is less stable at higher temperatures. NOxtrap catalysis is a cyclic operation. Metal compounds are believed toundergo a carbonate/nitrate conversion, as a dominant path, duringlean/rich operations. The sulfur poisoning of a NOx trap catalyst isdepicted below in Equations 6-7. In Equation 6, S occupies a site forNOx and in Equation 7, SOx replaces CO₃ or NOx.

Oxidation of NO to NO₂

NO+½O₂→NO₂  (1)

NOx Storage as Nitrate

2NO₂+MCO₃+½O₂→M(NO₃)₂+CO₂  (2)

NOx Release

M(NO₃)₂+2CO→MCO₃+NO₂+NO+CO₂  (3)

NOx Reduction to N₂

 NO₂+CO→NO+CO₂  (4)

2NO+2CO→N₂+2CO₂  (5)

SOx Poisoning Process

SO₂+½O₂→SO₃  (6)

SO₃+MCO₃→MSO₄+CO₂  (7)

In Equations 2, 3, and 7, M represents a divalent metal cation. M canalso be a monovalent or trivalent metal compound in which case theequations need to be rebalanced.

Comparative investigations on the currently most discussed lean burnDeNOx technologies comprising the continuously operating selectivelycatalytic reduction (SCR) of V-, Pt-, Ir-technologies as well as thediscontinuously operating NOx adsorption technology suggest that thelatter technology shows the most promising overall performance in termsof NOx, HC and CO removal in view of the proposed EURO III/IVlegislation. The sulfur concentration yields decisive influence on thelong-term activity of the NOx adsorption catalysts and it is shown by aworst case study, that even the use of low-sulfur fuel does not preventthe accumulation of sulfur on the NOx adsorption catalyst. Theaccumulation of sulfur on the catalyst has to be counteracted by anengine induced desulfation strategy, by which the sulfur is driven outof the NOx adsorption catalyst. This requires the provision of reducingexhaust gas at elevated temperature for a short period of time. Anoptimization of the desulfation parameters is mandatory in order tosuppress the formation of H₂S. It is conjectured that the thermaldegradation of the NOx adsorption catalyst proceeds via two differentdeactivation modes. The first one is based upon the loss of Ptdispersion and is accelerated by the presence of oxygen while the secondone can be traced back to the reaction between NOx storage componentsand the porous support material. Wolfgang Strehlau et al., Conference“Engine and Environment” 97.

Direct injection technology for diesel engines as well as for gasolineengines are the most favored ways to reduce the CO₂ emissions in thefuture. NOx adsorber technology for gasoline DI engines as well as forHSDI diesel engines is the favored technology to meet future emissionlimits. Adsorber catalysts have demonstrated their potential to meetfuture emission legislation levels on prototype basis for gasoline anddiesel engines. Improving the NOx adsorber technology and theintegration of the adsorber system into the powertrain for theintroduction into the European market is the challenge for the nearfuture.

Using a catalyst cannot reduce much NOx pollutant in a lean burncondition. Abating NOx by a catalyst is very difficult in a lean(oxidation) environment. So far, use of any known catalysts (without aNOx trap) has achieved only very low conversion %. In order to convertNOx to N2, a reducing atmosphere is required. In the normal workingcondition of a diesel or lean burn engine, there is no reducingatmosphere, but always a lean (oxidation) engine out gas flow. Undersuch lean condition, NOx goes through the exhaust, as a pollutant, withlittle or no reduction.

A NOx trap traps (adsorbs) NOx in a lean environment so that NOx isdepleted from the exhaust gas stream. However, once the limited capacityof the trap is used up, no more NOx will be trapped. A NOx trap isuseful only when it can be regenerated. For instance, barium oxide canbe used as a regenerable NOx trap. In lean (oxidation) environment,barium oxide continues to trap NOx and form nitrate until its capacitybeing used up. To regenerate the trap, the gas flow is switched to rich(reducing) for a short period of time, when NOx is desorbed quickly fromthe trap, forming N₂, and the trap is regenerated back to barium oxide.Engine management or a hydrocarbon injection can be used to create thisrich environment. After regeneration, the NOx trap (ideally) recoversits full capacity as a fresh trap. Thus, an ideal NOx trap continuouslyworks in the alternative lean/rich environment.

During the regeneration (rich) period, NOx is released but not out theexhaust as a pollutant. Because a catalyst is used in conjunction withthe trap, the released NOx is reduced to NOx is reduced to N₂ in thereducing (rich) environment. Therefore, in the regeneration period, notonly the NOx trap is regenerated, but also the released NOx is convertedto harmless N₂ and CO₂.

If no SOx appears in the gas stream, many NOx traps including bariumoxide, work well. Unfortunately, there is always SOx. Although theamount of Sox is much less than NOx (e.g. SOx:NOx=1:100) SOx willeventually poison the NOx trap. This because that SOx is much strongerreactant than NOx. Once SOx is absorbed on site of the trap, it sticks(forms sulfate). SOx competes for sites with NOx and the trap iseventually poisoned. In addition, sulfated barium NOx trap cannot beregenerated (sulfated barium oxide needs a much higher temperature toregenerate than that of a normal NOx regeneration process).

The underlying theory of hydrothermal synthesis and the apparatus usedis described in “Hydrothermal Synthesis”, George W. Morey, Journal ofthe American Ceramic Society, Sep. 1, 1953. By way of illustration, thedetermination of the compositions of coexisting gas and liquid phases inthe system H₂O—Na₂O—SiO₂ at 400° C. are discussed.

The hydrothermal synthesis of lead titanate is reported to involve thetreatment of lead and titanium containing aqueous feedstock attemperatures ranging from 200° C. to 300° C. at autogenous pressures.Suitable sources of lead and titanium can be the oxides, or metalhydrous oxides derived from perchlorates, nitrates, acetates oralkoxides. Titanium isopropoxide and lead acetate have been used tostudy the hydrothermal synthesis of lead titanate particles. Twomechanisms are involved in the synthesis of lead titanate particles: 1)dissolution of the hydrous metal oxide feedstock followed byprecipitation and, 2) in situ crystallization of the hydrous oxideparticles. “Hydrothermal Formation Diagram In The Lead Titanate System”D. J. Watson et al., Ceram. Trans, 1988, vol 1, Ceram. Powder Sci. 2,Pt. A, 154-162.

The advantages of the hydrothermal synthesis of Ba-polytitanates in theform of fine has been reported in connection with microwaveapplications. “Hydrothermal precipitation and characterization ofpolytitanates in the system BaO—TiO₂” T. R. N. Kutty et al., Journal OfMaterials Science Letters 7, 601-603(1988).

U.S. Pat. No. 3,963,630 (Yonezawa et al.) discloses a process forproducing crystalline powder of a composition represented by theformula, PbTi_(q)Zr_(p)O₃. In the formula, q and p represent moleratios, the sum of q and p is equal to 1.00, the mole ratios q is notsmaller than 0.45 and not greater than 0.90, the mole ration p is notsmaller than 0.10 and not greater than 0.55. The process comprises thesteps of preparing an acidic aqueous solution of the metallic elementsin the mole ratios given in the formula, neutralizing the aqueoussolution to provide a suspension of hydroxides of the metallic elements,subjecting the suspension in an autoclave under pressure to atemperature between 150° and 330° C. for time sufficient to produceprecipitate of the composition and of an average particle size between0.02 and 0.2 micron in a mother liquor, and separating the precipitatefrom the mother liquor.

U.S. Pat. No. 5,727,385 (Hepburn '385) discloses a catalyst system,located in the exhaust gas passage of a lean-burn internal combustionengine, useful for converting carbon monoxide, nitrogen oxides, andhydrocarbons present in nitrogen oxide catalyst being a transition metalselected from the group consisting of cooper, chromium, iron cobalt,nickel, iridium, cadmium, silver, gold platinum, manganese, and mixturesthereof loaded on a refractory oxide or exchanged into zeolite; an (2) anitrogen oxide (NOx) trap material which absorbs NOx when the exhaustgas flowing into the trap material is lean and releases the absorbed NOxwhen the concentration of oxygen in the inflowing exhaust gas islowered. The nitrogen oxide trap material is located downstream of thelean-burn nitrogen oxide catalyst in the exhaust gas passage such thatthe exhaust gases are exposed to the lean-burn catalyst prior to beingexposed to the nitrogen oxide trap material.

U.S. Pat. No. 5,750,082 (Hepburn et al. '082) discloses a nitrogen oxidetrap useful for tapping nitrogen oxide present in the exhaust gasesgenerated during lean-burn operation of an internal combustion engine.The trap comprises distinct catalyst phases: (a) a porous support loadedwith catalyst comprising 0.1 to 5 weight % platinum; and (b) anotherporous support loaded with 2 to 30 weight % catalyst of an alkalinemetal material selected from the group consisting of alkali metalelements and alkaline earth elements.

U.S. Pat. No. 5,753,192 (Dobson et al.) discloses a nitrogen oxide trapuseful for trapping nitrogen oxide present in an exhaust gas streamgenerated during lean-burn operation of an internal combustion of theexhaust gas is lowered. The trap comprises a porous support loaded metalselected from platinum, palladium, rhodium and mixtures thereof; (b)3.5-15 wt. % zirconium; and (c) 15-30 wt. % sulfate.

U.S. Pat. No. 5,758,489 (Hepburn et al. '489) discloses a nitrogen oxidetrap useful for trapping nitrogen oxide present in the exhaust gasesgenerated during lean burn operation of an internal combustion engine.The trap comprises a porous support; and catalysts comprising at least10 weight percent lithium and 0.2 to 4 weight percent platinum loaded onthe porous support.

U.S. Pat. No. 5,759,553 (Lott et al.) discloses a NOx adsorber materialcomprising an activated alkali metal-doped and copper-doped hydrouszirconium oxide material that adsorbs NOx in an oxidizing atmosphere anddesorbs NOx in a non-oxidizing atmosphere.

U.S. Pat. No. 5,910,097 (Boegner et al.) discloses an exhaust emissioncontrol system for an internal combustion engine. The system comprisestwo absorber parts arranged in parallel for alternate adsorption anddesorption of nitrogen oxides contained in an exhaust from an engine. Ameans for conducting the exhaust further downstream is provided emergingfrom one of the two absorber parts currently operated in the adsorptionmode and for recycling the exhaust emerging from the other of the twoadsorber parts operating in the desorption mode into an intake line ofthe engine. An oxidizing converter is located near the engine andupstream from the adsorber parts for oxidation of at least NO containedin the exhaust to NO₂. An exhaust line section is located upstream ofthe adsorber parts and is divided into a main line branch and a partialline branch parallel to the main line branch. The two adsorber parts areconnected by control valves to the main line branch and the partial linebranch such that the one adsorber part that is operating in theadsorption mode is fed by the exhaust stream from the main line branchand the other adsorber part that is operating in the desorption mode issupplied by the exhaust stream from partial line branch.

European patent application 589,393A2 discloses a method for purifyingand oxygen rich exhaust gas by simultaneously removing the carbonmonoxide, hydrocarbons, and nitrogen oxides contained in the exhaustgas. The method comprises bringing the oxygen rich exhaust gas intocontact with an exhaust gas purifying catalyst comprised of (i) at leastone noble metal selected from the group consisting of platinum andpalladium (ii) barium, and (iii) at least one metal selected from thegroup consisting of alkali metals, iron, nickel, cobalt, and magnesium,supported on a carrier composed of a porous substance.

European patent application 669,157A1 discloses a catalyst for purifyingexhaust gases. The catalyst comprises a heat resistant support; a porouslayer coated on the heat resistant support; a noble metal catalystingredient loaded on the porous layer; and an NOx storage componentselected from the group consisting of alkaline-earth metals, rare-earthelements and alkali metals, and loaded on the porous layer. The noblemetal catalyst ingredient and the NOx storage component are disposedadjacent to each other, and dispersed uniformly in the porous layer.

European patent application 764,459A2 discloses a nitrogen oxide trapuseful for trapping nitrogen oxide present in the exhaust gasesgenerated during lean-burn operation of an internal combustion engine.The trap comprises distinct catalyst phases (a) a first porous supportloaded with catalyst comprising 0.1 to 5 weight % platinum; and (b) asecond porous support loaded with 2 to 30 weight % catalyst of amaterial selected from the group consisting of alkali metal elements andalkaline earth elements.

European patent application 764,460A2 discloses a nitrogen oxide trapuseful for trapping nitrogen oxide present in the exhaust gasesgenerated during lean-burn operation of an internal combustion engine.The trap comprises a porous support; and catalysts consisting ofmanganese and potassium loaded on the porous support.

Laboratory and engine tests were carried out to describe the sulfureffect on the NOx adsorbers catalysts efficiency for gasoline lean burnengines. One aspect of the study dealt with the NOx storage efficiencyof the adsorber under laboratory conditions, especially regarding theSO2 gas phase concentration. The rate of sulfur storing is greatlyaffected by the SO2 gas concentration. While 6.5 hours are required togo from 70% NOx reduction to only 35% when the gas mixture contains 10ppm SOx, it takes 20 hours with 5 ppm of SOx and more than 60 hours witha 2 ppm SO2 condition. The relationship between the loss in NOx trapperformance and SO2 concentration appears to have an exponential shape.The same amount of sulfur (0.8% mass) is deposited onto the catalystwithin 10 hours with the feed gas containing 10 ppm of SO2 and within 50hours with 2 ppm SO2. Nevertheless, it was shown that the loss inNOx-trap efficiency is not the same in these two cases. The efficiencydecreased from 70% to 25% in the first case (with 10 ppm SO2) and from70% to only 38% in the second case (with 2 ppm SO2). The second aspectdescribes a parametric study on engine bench concerning the sulfureffect on NOx trap efficiency and the required conditions (temperature,air/fuel ratio) to obtain different rates of desulfation. For instance,after 70 hours, NOx efficiency decreased from 90% to 25% with a sulfurcontent in gasoline of 110 ppm. Complete regeneration requires variousdurations of desulfation depending on air/fuel ratio (lambda=1 to 0.95)and temperature conditions (950 to 750° C.). For example, completeregeneration occurs after several minutes at lambda=1 and several setsof ten seconds at lambda=0.95 at 650° C. Results show that sulfurcontent close to EURO III gasoline standards is the main obstacle forthe introduction of NOx adsorber catalyst in Europe. Impact of Sulfur onNOx Trap Catalyst Activity Study of the Regeneration Conditions, M.Guyon et al., Society of Automotive Engineers, 982607 (1998).

The conventional catalysts described above employing NO_(x) storagecomponents have the disadvantage under practical applications ofsuffering from long-term activity loss because of SO_(x) poisoning ofthe NO_(x) traps. The conventional NO_(x) trap components employed inthe catalysts tend to trap SO_(x) and form very stable sulfates whichrequire regeneration at 650° C. which is not practical for lowtemperature diesel exhaust. Accordingly, it is a continuing goal todevelop NOx trap catalysts which can reversibly trap SO_(x) present inthe gaseous stream and thereby prevent SO_(x) sulfur oxide poisoning ofthe NO_(x) trap and can be regenerated at moderate temperatures withrich pulses, rather than at high temperatures.

SUMMARY OF THE INVENTION

The present invention pertains to a method for removing NO_(x)contaminants from a SOx containing gaseous stream comprising the stepsof:

(1) providing a catalyst composite;

(2) in a sorbing period, passing a lean gaseous stream comprising NO_(x)and SO_(x) within a sorbing temperature range through the catalystcomposite to sorb at least some of the NO_(x) contaminants and therebyprovide a NO_(x) depleted gaseous stream exiting the catalyst compositeand to sorb and abate at least some of the SO_(x) contaminants in thegaseous stream and thereby provide a SO_(x) depleted gaseous streamexiting the catalyst composite;

(3) in a NO_(x) desorbing and abating period, changing the lean gaseousstream to a rich gaseous stream to thereby reduce and desorb at leastsome of the NO_(x) contaminants from the catalyst composite and therebyprovide a reduced NO_(x) enriched gaseous stream exiting the catalystcomposite;

(4) in a SOx desorbing period, changing the lean gaseous stream to arich gaseous stream and raising the temperature of the gaseous stream towithin a desorbing temperature range to thereby reduce and desorb atleast some of the SO_(x) contaminants from the catalyst composite andthereby regenerate the catalyst composite and provide a reduced SO_(x)enriched gaseous stream exiting the catalyst composite; and wherein thecatalyst composite comprises:

(a) a platinum component;

(b) a support; and

(c) a NOx sorbent component comprising a first metal oxide and a secondmetal oxide, wherein the metal in the first metal oxide is selected fromthe group consisting of aluminum, titanium, zirconium, silicon, andcomposites thereof, and the metal in the second metal oxide is selectedfrom the group consisting of Group IIA metals, Group III metals, GroupIV metals, rare earth metals, and transition metals; wherein the metalin the first metal oxide is different from the metal in the second metaloxide;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of the first and secondmetal oxides, or precursors thereof, or both, wherein the precursors ofthe first and second metal oxides are metal salts which when hydrolyzedproduce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

The present invention also pertains to a catalyst composite prepared bya hydrothermal synthesis method which comprises the steps of:

(i) providing an aqueous suspension or solution of a first metal oxideand a second metal oxide, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor; and

(iv) forming an admixture of the precipitate, a platinum component, anda support.

The present invention also pertains to a method of forming a catalystcomposite which comprises the steps of:

(i) providing an aqueous suspension or solution of a first metal oxideand a second metal oxide, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor; and

(iv) forming an admixture of the precipitate, a platinum component, anda support.

The present invention also pertains to a method of forming a catalystcomposite which comprises the steps of:

(1) forming an admixture of:

(a) a support; and

(b) a NOx sorbent component;

(2) combining a water-soluble or dispersible platinum component and theadmixture from step (1) with an aqueous liquid to form a solution ordispersion which is sufficiently dry to absorb essentially all of theliquid;

(3) forming a layer of the solution or dispersion on a substrate; and

(4) converting the platinum component in the resulting layer to awater-insoluble form;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of a first metal oxideand a second metal oxide, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

The sulfur tolerant NOx trap catalyst composites are highly effectivewith a sulfur containing fuel by trapping sulfur oxide contaminants inlean when conventional NOx trap catalysts tend to be poisoned, andrelease sulfur oxide contaminants in rich to regenerate the NOx trap.The sulfur tolerant NOx trap catalyst composites are particularlysuitable for diesel engines because the composites can be regenerated atmoderate temperatures with rich pulses, rather than at hightemperatures.

In a preferred embodiment, the present invention pertains to a methodfor removing NO_(x) contaminants from a SOx containing gaseous streamcomprising the steps of:

(1) providing a catalyst composite;

(2) in a sorbing period, passing a lean gaseous stream comprising NO_(x)and SO_(x) within a sorbing temperature range through the catalystcomposite to sorb at least some of the NO_(x) contaminants and therebyprovide a NO_(x) depleted gaseous stream exiting the catalyst compositeand to sorb and abate at least some of the SO_(x) contaminants in thegaseous stream and thereby provide a SO_(x) depleted gaseous streamexiting the catalyst composite;

(3) in a NO_(x) desorbing and abating period, changing the lean gaseousstream to a rich gaseous stream to thereby reduce and desorb at leastsome of the NO_(x) contaminants from the catalyst composite and therebyprovide a reduced NO_(x) enriched gaseous stream exiting the catalystcomposite;

(4) in a SO_(x) desorbing period, changing the lean gaseous stream to arich gaseous stream and raising the temperature of the gaseous stream towithin a desorbing temperature range to thereby reduce and desorb atleast some of the SO_(x) contaminants from the catalyst composite andthereby regenerate the catalyst composite and provide a reduced SO_(x)enriched gaseous stream exiting the catalyst composite; and wherein thecatalyst composite comprises:

(a) a platinum component;

(b) a support; and

(c) a NOx sorbent component comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of the constituents ofthe NOx sorbent component;

(ii) subjecting the suspension or solution of the constituents of theNOx sorbent component to a temperature from about 150° C. to about 300°C. in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

In another preferred embodiment, the present invention pertains to acatalyst composite prepared by a hydrothermal synthesis method whichcomprises the steps of:

(a) providing an aqueous suspension or solution of a NOx sorbentcomponent comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

(b) subjecting the suspension or solution of the constituents of the NOxsorbent component to a temperature from about 150° C. to about 300° C.in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(c) separating the precipitate from the mother liquor; and

(d) forming an admixture of the precipitate, a platinum component, and asupport.

In another preferred embodiment, the present invention pertains to amethod of forming a catalyst composite which comprises the steps of:

(a) providing an aqueous suspension or solution of a NOx sorbentcomponent comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

(b) subjecting the suspension or solution of the constituents of the NOxsorbent component to a temperature from about 150° C. to about 300° C.in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(c) separating the precipitate from the mother liquor; and

(d) forming an admixture of the precipitate, a platinum component, and asupport.

In another preferred embodiment, the present invention pertains to amethod of forming a catalyst composite which comprises the steps of:

(1) forming an admixture of:

(a) a support; and

(b) a NOx sorbent component comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(V) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

(2) combining a water-soluble or dispersible platinum component and theadmixture from step (1) with an aqueous liquid to form a solution ordispersion which is sufficiently dry to absorb essentially all of theliquid;

(3) forming a layer of the solution or dispersion on a substrate; and

(4) converting the platinum component in the resulting layer to awater-insoluble form;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of the constituents ofthe NOx sorbent component;

(ii) subjecting the suspension or solution of the constituents of theNOx sorbent component to a temperature from about 150° C. to about 300°C. in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

The preferred NOx sorbent component may further comprise BaO in anamount from about 0.5% to about 15%.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF

The present invention relates to sulfur tolerant catalyst compositesuseful for reducing contaminants in exhaust gas streams, especiallygaseous streams containing sulfur oxide contaminants. More specifically,the present invention is concerned with improved NOx trap catalysts foruse in diesel engines as well as lean burn gasoline engines. The sulfurtolerant NOx trap catalyst composites comprise a platinum component, asupport, and a NOx sorbent component prepared by hydrothermal synthesis.The NOx sorbent component comprises a first metal oxide and a secondmetal oxide. The metal in the first metal oxide is selected from thegroup consisting of aluminum, titanium, zirconium, silicon, andcomposites thereof, and the metal in the second metal oxide is selectedfrom the group consisting of Group IIA metals, Group III metals, GroupIV metals, rare earth metals, and transition metals. The metal in thefirst metal oxide is different from the metal in the second metal oxide.The sulfur tolerant NOx trap catalyst composites are highly effectivewith a sulfur containing fuel by trapping sulfur oxide contaminantswhich tend to poison conventional NOx trap catalysts used to abate otherpollutants in the stream. The sulfur tolerant NOx trap catalystcomposites are suitable for diesel engines because the composites can beregenerated at moderate temperatures with rich pulses, rather than athigh temperatures. Conventional NOx trap catalysts are readily poisonedby sulfur and cannot be regenerated by rich pulses below 650° C. Sincethe exhaust temperature of diesel engines is low, the temperaturerequirement of 650° C. for regeneration and higher is not practical. Thesulfur tolerant NOx trap catalyst composites of the present inventioncan be regenerated with rich pulses at moderate temperatures (550° C. orlower). Applicants believe that when the NOx sorbent components of thepresent invention are prepared by hydrothermal synthesis, the resultingNOx sorbent components are fine crystalline powders having sufficientlysmall average particle sizes (microparticles) such that the NOx sorbentcomponents regenerate (desorb SOx) faster than NOx sorbent componentsnot prepared by hydrothermal synthesis.

The improved NOx trap catalysts are sulfur tolerant and can be employedin a lean/rich environment containing sulfur for an extended period oftime. Accumulated sulfur in the NOx trap catalysts can thereafter bereleased at a temperature that is easily reachable under the engineoperation conditions, for example, 550° C. or lower. Thus, the NOx trapcatalysts can be used in a sulfur containing environment continuouslyprovided they are regenerated periodically. If sulfur can be releasedunder normal engine working conditions, then sulfur is automaticallybeing regenerated without additional efforts and the NOx trap catalystbecomes a truly sulfur resistant NOx trap.

The NOx trap materials of the present invention are generated by ahydrothermal synthesis process so as to yield ultra fine sub-micronmixed oxide composites. While not wishing to be held to a mechanism, itis believed that ultra fine sub-micron mixed oxides are more active thanthe large, greater than 1 micron size, bulk phase materials that areproduced by conventional processes. It is further believed that thereductant can more effectively clean the surface to regenerate newadsorption sites, onto which nitrates and sulfates adsorb and the bulkoxide takes longer to regenerate because of diffusion limitation in therapid regeneration needed. On a diesel engine, a bulk NOx trap compositecan not be fully ‘scrubbed’ of sulfur in the short regeneration cycle.Therefore, it becomes quickly poisoned. Conversely, the oxides producedfrom the hydrothermal method regenerate more effectively.

A fresh SOx trap should clear SOx before it reaches the NOx trap locateddownstream. However, any SOx trap has limited capacity. Many problemsexist when a separated SOx trap is used together with a NOx trap. Ahigher temperature is needed to regenerate the SOx trap, the releasedSOx in the gas flow must be avoided to poison the NOx trap that islocated down stream, engine management becomes more complicated, and thepollutant abatement system becomes complicated. The present inventionprovides a trap that not only traps NOx and SOx but also releases SOxwhen NOx is released during the NOx regeneration so that some sites thathave been trapping SOx are also regenerated at the same time. In otherwords, the SOx tolerant NOx trap may not require a higher temperature tobe regenerated after being sulfated in the SOx environment. As anoption, a separate SOx trap is not needed, the NOx and SOx trap may becombined. A lean burn or diesel catalyst equipped with a NOx/SOx trapcan be regenerated similar to regenerating a simple NOx trap in a sulfurfree environment.

With the SOx tolerant NOx trap of the present invention, one may operatethe pollutant abating system without the need of a higher temperaturefor SOx desorbing, and/or a separate SOx desorbing period. The SOxtolerant NOx trap can also work continuously at a moderate temperate(e.g. 300° C.) without raising the temperature for trap regeneration(only lean/rich alternative required).

The present invention includes a method of making SOx tolerant NOx trapsthrough the selection of the right mixed oxides prepared by hydrothermalsynthesis. After being sulfated, the SOx tolerant NOx trap can be fullyregenerated at a temperature as low as 400° C. Barium oxide type NOxtraps cannot be regenerated at such a low temperature. After beingextensively sulfated for a continuous 68 hours (the amount of SOx passedthrough equaled the same amount of the trap material), the NOxconversion was still effective. The SOx tolerant NOx trap workedeffectively without any temperature rising for trap regeneration (onlylean/rich alternative required).

A catalyst adsorbs or traps NOx when the exhaust gas is lean andreleases the stored NOx when the exhaust stream is rich. The releasedNOx is subsequently reduced to N₂ over the same catalyst. The richenvironment in a diesel engine is normally realized with a rich pulsegenerated by either engine management or injection of reducing agents(such as fuel, or a CO or CO/H₂ mixture) into the exhaust pipe. Thetiming and frequency of the rich pulse is determined by the NOx levelemitted from the engine, the richness of the exhaust, or theconcentration of the reductant in the rich pulse and the NOx conversiondesired. Normally, the longer the lean period, the longer the rich pulseis needed. The need for longer rich pulse timing may be compensated byhigher concentration of the reductant in the pulse. Overall, thequantity of the NOx trapped by the NOx trap should be balanced by thequantity of the reductant in the rich pulse. The lean NOx trapping andrich NOx trap regeneration are operative at normal diesel operatingtemperatures (150-450° C.). Beyond this temperature window, theefficiency of the NOx trap catalyst becomes less effective. In a sulfurcontaining exhaust stream, the catalyst becomes deactivated over timedue to sulfur poisoning. To regenerate the sulfur-poisoned NOx trap, arich pulse (or pulses) must be applied at temperatures higher than thenormal diesel operating temperature. The regeneration time of thegeneration depends on the sulfur level in the exhaust (or fuel sulfurlevel) and the length of the catalyst had exposed to thesulfur-containing stream. The quantity of the reductant added during thedesulfation should counterbalance the total amount of sulfur trapped inthe catalyst. Engine operability will determine whether a single longpulse or multiple short pulses are employed.

As used herein, the following terms, whether used in singular or pluralform, have the meaning defined below.

The term “catalytic metal component”, or “platinum metal component”, orreference to a metal or metals comprising the same, means acatalytically effective form of the metal or metals, whether the metalor metals are present in elemental form, or as an alloy or a compound,e.g., an oxide.

The term “component” or “components” as applied to NO_(x) sorbents meansany effective NO_(x)-trapping forms of the metals, e.g., oxygenatedmetal compounds such as metal hydroxides, mixed metal oxides, metaloxides or metal carbonates.

The term “composite” means bimetallic or multi-metallic oxygencompounds, such as Ba₂SrWO₆, which are true compounds as well asphysical mixtures of two or more individual metal oxides, such as amixture of SrO and BaO.

The term “dispersed”, when applied to a component dispersed onto a bulksupport material, means immersing the bulk support material into asolution or other liquid suspension of the component or a precursorthereof. For example, the sorbent strontium oxide may be dispersed ontoan alumina support material by soaking bulk alumina in a solution ofstrontium nitrate (a precursor of strontia), drying the soaked aluminaparticles, and heating the particles, e.g., in air at a temperature fromabout 450° C. to about 750° C. (calcining) to convert the strontiumnitrate to strontium oxide dispersed on the alumina support materials.

The term “gaseous stream” or “exhaust gas stream” means a stream ofgaseous constituents, such as the exhaust of an internal combustionengine, which may contain entrained non-gaseous components such asliquid droplets, solid particulates, and the like.

The terms “g/in³” or “g/ft³” are used to describe weight per volumeunits describe the weight of a component per volume of catalyst or trapmember including the volume attributed to void spaces such as gas-flowpassages.

The term “lean” mode or operation of treatment means that the gaseousstream being treated contains more oxygen that the stoichiometric amountof oxygen needed to oxidize the entire reductants content, e.g., HC, COand H₂, of the gaseous stream.

The term “platinum group metals” means platinum, rhodium, palladium,ruthenium, iridium, and osmium.

The term “sorb” means to effect sorption.

The term “stoichiometric/rich” mode or operation of treatment means thatthe gaseous stream being treated refers collectively to thestoichiometric and rich operating conditions of the gas stream.

The term “washcoat” has its usual meaning in the art of a thin, adherentcoating of a catalytic or other material applied to a refractory carriermaterial, such as a honeycomb-type carrier member, which is sufficientlyporous to permit the passage there through of the gas stream beingtreated.

In accord with the present invention, a method is provided for removingNO_(x) contaminants from a SOx containing gaseous stream. The methodcomprises (1) providing a catalyst composite; (2) in a sorbing period,passing a lean gaseous stream comprising NO_(x) and SO_(x) within asorbing temperature range through the catalyst composite to sorb atleast some of the NO_(x) contaminants and thereby provide a NO_(x)depleted gaseous stream exiting the catalyst composite and to sorb andabate at least some of the SO_(x) contaminants in the gaseous stream andthereby provide a SO_(x) depleted gaseous stream exiting the catalystcomposite; (3) in a NO_(x) desorbing and abating period, changing thelean gaseous stream to a rich gaseous stream to thereby reduce anddesorb at least some of the NO_(x) contaminants from the catalystcomposite and thereby provide a reduced NO_(x) enriched gaseous streamexiting the catalyst composite; (4) in a SO_(x) desorbing period,changing the lean gaseous stream to a rich gaseous stream and raisingthe temperature of the gaseous stream to within a desorbing temperaturerange to thereby reduce and desorb at least some of the SO_(x)contaminants from the catalyst composite and thereby regenerate thecatalyst composite and provide a reduced SO_(x) enriched gaseous streamexiting the catalyst composite. The catalyst composite comprises (a) aplatinum component; (b) a support; and (c) a NOx sorbent componentcomprising a first metal oxide and a second metal oxide, wherein themetal in the first metal oxide is selected from the group consisting ofaluminum, titanium, zirconium, silicon, and composites thereof, and themetal in the second metal oxide is selected from the group consisting ofGroup IIA metals, Group III metals, Group IV metals, rare earth metals,and transition metals; wherein the metal in the first metal oxide isdifferent from the metal in the second metal oxide. The NOx sorbentcomponent is prepared by a hydrothermal synthesis process comprising thesteps of (i) providing an aqueous suspension or solution of the firstand second metal oxides, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides; (ii) subjecting thesuspension or solution of the first and second metal oxides, orprecursors thereof, to a temperature from about 150° C. to about 300° C.in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and (iii) separating the precipitate fromthe mother liquor.

In general, the SOx desorbing temperature range is greater than about300° C., preferably, the desorbing temperature range is greater thanabout 350° C., more preferably, the desorbing temperature range isgreater than about 400° C., and most preferably the desorbingtemperature range is greater than about 450° C. The SOx desorbingtemperature may also be greater than about 500° C., preferably greaterthan about 550° C., more preferably greater than about 600° C., and mostpreferably greater than about 650° C.

As set out above, the sulfur tolerant catalyst composite of the presentinvention includes a platinum component, and optionally a platinum groupmetal component other than platinum. The optional platinum group metalcomponent other than platinum may be selected from the group consistingof palladium, rhodium, ruthenium, iridium, and osmium components. Thepreferred platinum group metal component other than platinum is selectedfrom the group consisting of palladium, rhodium, and mixtures thereof.

The sulfur tolerant catalyst composite of the present invention alsoincludes a support made of a high surface area refractory oxide support.The support may be selected from the group consisting of alumina,silica, titania, and zirconia compounds. Useful high surface areasupports include one or more refractory oxides. These oxides include,for example, silica and metal oxides such as alumina, including mixedoxide forms such as silica-alumina, aluminosilicates which may beamorphous or crystalline, alumina-zirconia, alumina-chromia,alumina-ceria and the like. Preferably the support is an activatedcompound selected from the group consisting of activated alumina,alumina-ceria, alumina-chromia, alumina-silica, alumina-zirconia,silica, silica-titania, silica-titania-alumina, silica-titania-zirconia,titania, zirconia, zirconia-titania, and zirconia-alumina-titania.Desirably, the active alumina has a specific surface area of 60 to 300m²/g.

The sulfur tolerant catalyst composite of the present invention alsoincludes a NO_(x) sorbent component. The NOx sorbent component comprisesa first metal oxide and a second metal oxide. The metal in the firstmetal oxide is selected from the group consisting of aluminum, titanium,zirconium, silicon, and composites thereof. The metal in the secondmetal oxide is selected from the group consisting of Group IIA metals,Group III metals, Group IV metals, rare earth metals, and transitionmetals. The metal in the first metal oxide is different from the metalin the second metal oxide. Preferably, the metal in the second metaloxide is selected from the group consisting of magnesium, calcium,strontium, barium, scandium, titanium, zirconium, hafnium, lanthanum,cerium, praseodymium, neodymium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium, ytterbium, vanadium,chromium, manganese, iron, cobalt, nickel, copper, and zinc. Morepreferably, the metal in the second metal oxide is selected from thegroup consisting of barium, lanthanum, cerium, iron, cobalt, and copper.In a specific embodiment, the metals in the first and second metaloxides are selected from the group consisting of aluminum/cerium/barium,aluminum/copper/lanthanum, aluminum/cobalt/lanthanum, and iron/aluminum.

As set out above, the NOx sorbent component is prepared by ahydrothermal synthesis process which employs an aqueous suspension orsolution of the first and second metal oxides, or precursors thereof, orboth. The precursors of the first and second metal oxides are metalsalts when hydrolyzed produce the respective metal oxides. Preferredprecursors of the first metal oxide and second metal oxide arewater-soluble/dispersible metal salts selected from the group consistingof acetates, nitrates, hydroxides, oxychlorides, hydroxychlorides,carbonates, sulfates, oxalates, and tartrates.

The metal in the first metal oxide and the metal in the second metaloxide are preferably present in a ratio from about 3:7 to about 9:1,more preferably from about 4:6 to about 8.5:1.5, respectively, and mostpreferably from about 5:5 to about 7.5:2.5, respectively.

As set out above, the NOx sorbent component is prepared by hydrothermalsynthesis. In general, the process comprises the steps of (i) providingan aqueous suspension or solution of the at least 2 metal oxides, orprecursors thereof, wherein the precursors of the at least 2 metaloxides are metal salts which when hydrolyzed produce the respectivemetal oxides; (ii) subjecting the suspension or solution of the at least2 metal oxides, or precursors thereof, to a temperature from about 150°C. to about 300° C. in an autoclave under pressure for a time sufficientto produce a precipitate having an average particle size from about0.001 to about 0.2 micron in a mother liquor; and (iii) separating theprecipitate from the mother liquor.

The suspension of metal hydroxides is preferably subjected to atemperature from about 175° C. to about 250° C., preferably from about185° C. to about 240° C. The precipitate preferably has an averageparticle size from about 0.01 to about 0.2 micron, preferably from about0.05 to about 0.15 micron.

In a preferred embodiment, the catalyst composite comprises (i) at leastabout 1 g/ft³ of the platinum component; (ii) from about 0.15 g/in³ toabout 6.0 g/in³ of the support; and (iii) from about 0.025 g/in³ toabout 4 g/in³ of the NO_(x) sorbent component.

The catalyst composite may be supported on a metal or ceramic honeycombcarrier or is self-compressed.

The NOx sorbent component may further comprise a third metal oxide,wherein the metal in the third metal oxide is selected from the groupconsisting of aluminum, titanium, zirconium, silicon, and compositesthereof, and the metal in the second metal oxide is selected from thegroup consisting of Group IIA metals, Group III metals, Group IV metals,rare earth metals, and transition metals. In this embodiment, the metalin the third metal oxide is different from the metal in the first andsecond metal oxides.

In accordance with the present invention, NOx sorbent components areprepared by hydrothermal synthesis to yield fine powders. In general,the NOx sorbent components are prepared by providing an aqueoussuspension or solution of the first and second metal oxides, orprecursors thereof, or both, subjecting the suspension or solution ofthe first and second metal oxides, or precursors thereof, to atemperature from about 150° C. to about 300° C. in an autoclave underpressure for a time sufficient to produce a precipitate having anaverage particle size from about 0.001 to about 0.2 micron in a motherliquor, and separating the precipitate from the mother liquor.

The precursor solution/suspension is subsequently subjected tohydrothermal reaction with stirring to provide a fine precipitate. Thereaction may be carried out in an autoclave, which is preferably made ofstainless steel coated with heat-resistant high polymer, such aspolytetrafluoroethylene. The precipitate produced by the hydrothermalreaction at a temperature from about 150° C. to about 300° C. generallyhas an average particle size between 0.001 and 0.2 micron. Theprecipitate may be separated from the mother liquor by filtration, thenwashed and dried.

In a preferred embodiment, the NOx sorbent component is prepared by aprocess comprising the steps of (i) providing an aqueous suspension orsolution of the at least 2 metal oxides, or precursors thereof, or both,wherein the precursors of the at least 2 metal oxides are metal saltswhich when hydrolyzed produce the respective metal oxides; (ii)subjecting the suspension or solution of the at least 2 metal oxides, orprecursors thereof, to a temperature from about 150° C. to about 300° C.in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and (iii) separating the precipitate fromthe mother liquor.

In use, the exhaust gas stream, comprising air, water, hydrocarbons,carbon monoxide, nitrogen oxides, and sulfur oxides and which iscontacted with the catalyst composite of the present invention, isalternately adjusted between lean and stoichiometric/rich operatingconditions so as to provide alternating lean operating periods andstoichiometric/rich operating periods. The exhaust gas stream beingtreated may be selectively rendered lean or stoichiometric/rich eitherby adjusting the air-to-fuel ratio fed to the engine generating theexhaust or by periodically injecting a reductant into the gas streamupstream of the catalyst. For example, the catalyst composite of thepresent invention is well suited to treat the exhaust of engines,especially diesel engines, which continuously run lean. In a dieselengine, in order to establish a stoichiometric/rich operating period, asuitable reductant, such as fuel, may be periodically sprayed into theexhaust immediately upstream of the catalytic trap of the presentinvention to provide at least local (at the catalytic trap)stoichiometric/rich conditions at selected intervals. Partial lean-burnengines, such as partial lean-burn gasoline engines, are designed withcontrols which cause them to operate lean with brief, intermittent richor stoichiometric conditions. In practice, the sulfur tolerant NOx trapcatalyst composite absorbs in-coming SO_(x) during a lean mode operation(100° C. to 500° C.) and desorbs SO_(x) (regenerate) during a rich modeoperation (greater than about 300° C., preferably greater than about350° C., more preferably greater than about 400° C., and most preferablygreater than about 450° C. The SOx desorbing temperature may also begreater than about 500° C., preferably greater than about 550° C., morepreferably greater than about 600° C., and most preferably greater thanabout 650° C. When the exhaust gas temperature returns to a lean modeoperation (100° C. to 500° C.), the regenerated sulfur tolerant NOx trapcatalyst composite will again selectively absorb in-coming SO_(x). Theduration of the lean mode may be controlled so that the sulfur tolerantNOx trap catalyst composite will not be saturated with SO_(x).

When the composition is applied as a thin coating to a monolithiccarrier substrate, the proportions of ingredients are conventionallyexpressed as grams of material per cubic inch (g/in³) of the catalystand the substrate. This measure accommodates different gas flow passagecell sizes in different monolithic carrier substrates. Platinum groupmetal components are based on the weight of the platinum group metal.

A useful and preferred sulfur tolerant NOx trap catalyst composite hasat least about 1 g/ft³ of a platinum component; from about 0.15 g/in³ toabout 4.0 g/in³ of a support; at least about 1 g/ft³ of a platinum groupmetal component other than platinum; from about 0.025 g/in³ to about 4g/in³ of a NO_(x) sorbent component.

The specific construction of the catalyst composite set out aboveresults in an effective catalyst that reversibly traps sulfur oxidecontaminants present and thereby prevents the sulfur oxide contaminantsfrom poisoning the NOx trap catalysts for use in diesel engines. Thecatalyst composite can be in the form of a self-supported article suchas a pellet, and more preferably, the sulfur tolerant NOx trap catalystcomposite is supported on a carrier, also referred to as a substrate,preferably a honeycomb substrate. A typical so-called honeycomb-typecarrier member comprises a material such as cordierite or the like,having a plurality of fine, gas-flow passages extending from the frontportion to the rear portion of the carrier member. These fine gas-flowpassages, which may number from about 100 to 900 passages or cells persquare inch of face area (“cpsi”), have a catalytic trap material coatedon the walls thereof.

The present invention also includes a method for treating an exhaust gasstream which comprises the step of contacting the gas stream comprisingcarbon monoxide and/or hydrocarbons, nitrogen oxides, and sulfur oxideswith the catalyst composite set out above. The present invention alsoincludes a method of treating an exhaust gas stream comprising the stepsof contacting the stream with the catalyst composite set out above underalternating periods of lean and stoichiometric or rich operation.Contacting is carried out under conditions whereby at least some of theSO_(x) in the exhaust gas stream is trapped in the catalytic materialduring the periods of lean operation and is released and reduced duringthe periods of stoichiometric or rich operation.

In a specific embodiment, the present invention pertains to a catalystcomposite prepared by a hydrothermal synthesis method which comprisesthe steps of:

(i) providing an aqueous suspension or solution of a first metal oxideand a second metal oxide, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor; and

(iv) forming an admixture of the precipitate, a platinum component, anda support.

The present invention also pertains to a method of forming a catalystcomposite which comprises the steps of:

(i) providing an aqueous suspension or solution of a first metal oxideand a second metal oxide, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor; and

(iv) forming an admixture of the precipitate, a platinum component, anda support.

The present invention also pertains to a method of forming a catalystcomposite which comprises the steps of:

(1) forming an admixture of:

(a) a support; and

(b) a NOx sorbent component;

(2) combining a water-soluble or dispersible platinum component and theadmixture from step (1) with an aqueous liquid to form a solution ordispersion which is sufficiently dry to absorb essentially all of theliquid;

(3) forming a layer of the solution or dispersion on a substrate; and

(4) converting the platinum component in the resulting layer to awater-insoluble form;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of a first metal oxideand a second metal oxide, or precursors thereof, or both, wherein theprecursors of the first and second metal oxides are metal salts whichwhen hydrolyzed produce the respective metal oxides;

(ii) subjecting the suspension or solution of the first and second metaloxides, or precursors thereof, to a temperature from about 150° C. toabout 300° C. in an autoclave under pressure for a time sufficient toproduce a precipitate having an average particle size from about 0.001to about 0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

The sulfur tolerant NOx trap catalyst composites are highly effectivewith a sulfur containing fuel by trapping sulfur oxide contaminantswhich tend to poison conventional NOx trap catalysts used to abate otherpollutants in the stream. The sulfur tolerant NOx trap catalystcomposites are suitable for diesel engines because the composites can beregenerated at moderate temperatures with rich pulses, rather than athigh temperatures.

The sulfur tolerant catalyst composite may optionally compriseconventional components known in the art.

In order to deposit the coat slurries on a macrosized carrier, one ormore comminuted slurries are applied to the carrier in any desiredmanner. Thus the carrier may be dipped one or more times in the slurry,with intermediate drying if desired, until the appropriate amount ofslurry is on the carrier. The slurry employed in depositing thecatalytically-promoting metal component-high area support composite onthe carrier will often contain about 20% to 60% by weight offinely-divided solids, preferably about 25% to 55% by weight.

The sulfur tolerant catalyst composite of the present invention can beprepared and applied to a suitable substrate, preferably a metal orceramic honeycomb carrier, or may be self-compressed. The comminutedcatalytically-promoting metal component-high surface area supportcomposite can be deposited on the carrier in a desired amount, forexample, the composite may comprise about 2% to 40% by weight of thecoated carrier, and is preferably about 5% to 30% by weight for atypical ceramic honeycomb structure. The composite deposited on thecarrier is generally formed as a coating over most, if not all, of thesurfaces of the carrier contacted. The combined structure may be driedand calcined, preferably at a temperature of at least about 250° C. butnot so high as to unduly destroy the high area of the refractory oxidesupport, unless such is desired in a given situation.

The carriers useful for the catalysts made by this invention may bemetallic in nature and be composed of one or more metals or metalalloys. The metallic carriers may be in various shapes such ascorrugated sheet or in monolithic form. Preferred metallic supportsinclude the heat-resistant, base-metal alloys, especially those in whichiron is a substantial or major component. Such alloys may contain one ormore of nickel, chromium, and aluminum, and the total of these metalsmay advantageously comprise at least about 15% by weight of the alloy,for instance, about 10% to 25% by weight of chromium, about 3% to 8% byweight of aluminum and up to about 20% by weight of nickel, say at leastabout 1% by weight of nickel, if any or more than a trace amount bepresent. The preferred alloys may contain small or trace amounts of oneor more other metals such as manganese, copper, vanadium, titanium andthe like. The surfaces of the metal carriers may be oxidized at quiteelevated temperatures, e.g. at least about 1000° C., to improve thecorrosion resistance of the alloy by forming an oxide layer on thesurface of carrier which is greater in thickness and of higher surfacearea than that resulting from ambient temperature oxidation. Theprovision of the oxidized or extended surface on the alloy carrier byhigh temperature oxidation may enhance the adherence of the refractoryoxide support and catalytically-promoting metal components to thecarrier.

Any suitable carrier may be employed, such as a monolithic carrier ofthe type having a plurality of fine, parallel gas flow passagesextending there through from an inlet or an outlet face of the carrier,so that the passages are open to fluid flow there through. The passages,which are essentially straight from their fluid inlet to their fluidoutlet, are defined by walls on which the catalytic material is coatedas a “washcoat” so that the gases flowing through the passages contactthe catalytic material. The flow passages of the monolithic carrier arethin-walled channels which can be of any suitable cross-sectional shapeand size such as trapezoidal, rectangular, square, sinusoidal,hexagonal, oval, circular. Such structures may contain from about 60 toabout 600 or more gas inlet openings (“cells”) per square inch of crosssection. The ceramic carrier may be made of any suitable refractorymaterial, for example, cordierite, cordierite-alpha alumina, siliconnitride, zircon mullite, spodumene, alumina-silica magnesia, zirconsilicate, sillimanite, magnesium silicates, zircon, petalite, alphaalumina and aluminosilicates. The metallic honeycomb may be made of arefractory metal such as a stainless steel or other suitable iron basedcorrosion resistant alloys.

The substrate can comprise a monolithic honeycomb comprising a pluralityof parallel channels extending from the inlet to the outlet. Themonolith can be selected from the group of ceramic monoliths andmetallic monoliths. The honeycomb can be selected from the groupcomprising flow through monoliths and wall flow monoliths. Suchmonolithic carriers may contain up to about 700 or more flow channels(“cells”) per square inch of cross section, although far fewer may beused. For example, the carrier may have from about 60 to 600, moreusually from about 200 to 400, cells per square inch (“cpsi”). Thesulfur tolerant catalyst composite can be coated in layers on amonolithic substrate generally which can comprise from about 0.50 g/in³to about 6.0 g/in³, preferably about 5.0 g/in³ to about 5.0 g/in³ ofcatalytic composition based on grams of composition per volume of themonolith.

The present invention includes a method comprising passing an inlet endfluid comprising an inlet end coating composition into a substrate asrecited above. For the purpose of the present invention a fluid includesliquids, slurries, solutions, suspensions and the like. The aqueousliquid passes into the channel inlets and extending for at least part ofthe length from the inlet end toward the outlet end to form an inlet endlayer coating, with at least one inlet end coating extending for onlypart of the length from the inlet end toward the outlet end. A vacuum isapplied to the outlet end while forcing a gas stream through thechannels from the inlet end after the formation of each inlet endcoating without significantly changing the length of each inlet layercoating. At least one outlet end aqueous fluid comprising at least oneoutlet end coating composition is passed into the substrate through theat least some of the channel outlets at the substrate outlet end. Theaqueous liquid passes into the channels and extending for at least partof the length from the outlet end toward the inlet end to form at leastone outlet end layer coating. The method can further comprise applying avacuum to the inlet end while forcing a gas stream through the channelsfrom the outlet end after the formation of each outlet end coatingwithout significantly changing the length of each outlet layer coating.

The method can further comprise the step of fixing the precious metalcomponent selected from the inlet precious metal component of the inletlayer and the outlet precious metal component of the outlet layer to therespective inlet or outlet component selected from the inlet refractoryoxide and inlet rare earth metal oxide components and the outletrefractory oxide and outlet rare earth metal oxide components. Thefixing can be conducted prior to coating the inlet and outlet layers.The step of fixing can comprise chemically fixing the precious metalcomponent on the respective refractory oxide and/or rare earth metaloxide. Alternatively, the step of fixing can comprise thermally treatingthe precious metal component on the respective refractory oxide and/orrare earth metal oxide. The step of fixing comprises calcining theprecious metal component on the respective refractory oxide and/or rareearth metal oxide. The step of calcining can be conducted at from 200°C., preferably 250° C. to 900° C. at from 0.1 to 10 hours. The steps ofthermally fixing each layer are preferably conducted after coating andprior to coating a subsequent layer. The step of thermally treating thesubstrate upon completion of coating all layers at from 200° C. to 400°C. at from 1 to 10 seconds. The step of calcining is preferably thesubstrate conducted upon completion of coating all layers. The step ofcalcining is conducted at from 250° C. to 900° C. at from 0.1 to 10hours.

Preferably, the precious metal can be prefixed on the supports.Alternatively the method further comprises fixing the soluble componentsin the layer such as one precious metal component to one of therefractory oxide or rare earth metal oxide components, the fixing beingconducted prior to coating the layers. The step of fixing can comprisechemically fixing the precious metal on the respective refractory oxideand/or rare earth metal oxide. More preferably, the step of fixingcomprises thermally treating the precious metal on the refractory oxideand/or rare earth metal oxide. The step of thermally treating thesubstrate upon completion of coating one or more layers at from 200° C.to 400° C. at from 1 to 10, and preferably 2 to 6 seconds. The heat isprovided by forcing a gas stream, preferably air which is heated to from200° C. to 400° C. This temperature range has been found tosubstantially fix the soluble components such as precious metalcomponents. The combination of flow rate and temperature of the gasstream should be sufficient to heat the coating layer and preferably,providing a minimum of heat to the underlying substrate to enable rapidcooling in the subsequent cooling step prior to application ofsubsequent layers. Preferably, the steps of thermally fixing each layer,preferably followed by cooling with ambient air, are conducted aftercoating and prior to coating a subsequent layer. The cooling step ispreferably conducted using ambient air typically at from 5° C. to 40° C.at from 2 to 20, and preferably 4 to 10 seconds at a suitable flow rate.The combination of the ambient air flow rate and temperature of the gasstream should be sufficient to cool the coating layer. This methodpermits continuous coating of a plurality of layers on a substrate toform the above described article of the present invention. A preferredmethod comprises the step of fixing the precious metal component to therefractory oxide and rare earth metal oxide components, the fixing beingconducted prior to coating the first and second layers.

In yet another embodiment the method comprises the step of applying avacuum to the partially immersed substrate at an intensity and a timesufficient to draw the coating media upwardly to a predesignateddistance from the bath into each of the channels to form a uniformcoating profile therein for each immersion step. Optionally, andpreferably the substrate can be turned over to repeat the coatingprocess from the opposite end. The coated substrate should be thermallyfixed after forming the layer.

The method can include a final calcining step. This can be conducted inan oven between coating layers or after the coating of all the layers onthe substrate has been completed. The calcining can be conducted at from250° C. to 900° C. at from 0.1 to 10 hours and preferably from 450° C.to 750° C. at from at from 0.5 to 2 hours. After the coating of alllayers is complete the substrate can be calcined.

A method aspect of the present invention provides a method for treatinga gas containing noxious components comprising one or more of carbonmonoxide, hydrocarbons and nitrogen oxides, by converting at least someof each of the noxious components initially present to innocuoussubstances such as water, carbon dioxide and nitrogen. The methodcomprises the step of contacting the gas under conversion conditions(e.g., a temperature of about 100° C. to 950° C. of the inlet gas to thecatalyst composition) with a catalyst composition as described above.

In a specific embodiment, the present invention pertains to a phase witha hematite type composition and structure (assay 7). The material iscomposed of a mixture of iron oxide, aluminum oxide, and small amountsof both yttrium oxide and strontium oxide. The iron oxide phase is notan exact match for hematite. The diffraction pattern is not a perfectmatch, slight shifts in peak position are noticed when compared to areference pattern of hematite. The work is directed at trying tomanipulate the physical and or catalytic properties of simple oxides byaltering their structures. Small differences in composition or structurecan be the cause of changes in these properties or the development ofnew ones. An example would be the generation of a polarized structure inBaTiO₃ upon its distortion from a cubic to a tetragonal structure, thusgiving it valuable dielectric properties. This work centers on trying todetermine if a small structural change can be measured as a small changein lattice constants. To do this an accurate determination of the unitcell constants is needed. It is also important to know the errorassociated with the measurement to know when two measured latticeparameters are really different.

Hematite is a primitive hexagonal cell. To determine the unit cellparameters, XRD data was collected on a NIST Sibley Ore sample (assayD20327) with White Rock 10 μm quartz as the internal standard. XRD dataon an experimental sample (assay 7) was also collected using the samemethod. Both data sets were refined to obtain the unit cell parameters,a_(o) and c_(o). Results are tabulated below along with ICDD data,reference card number 33-664.

sample a_(o)(A) c_(o)(A) Assay 7 5.00821 13.69433 experimental(0.001166) (0.002883) NIST hematite std 5.03453 13.75294 (0.001166)(0.00009) ICDD 5.036 13.749 No standard reference 33-664 deviations aregiven

The a_(o) and c_(o) values of the NIST hematite standard sample closelymatch those of the ICDD hematite reference card 33-664, although nostandard deviations are given in the ICDD card. The close match of theexperimental data with that of ICDD is a strong indication that the datacollection and refinement are acceptable. Thus, a valid comparison canbe made between cell parameters of the sample assay 7 and those of NISThematite sample to determine whether the crystal structure of the assay7 is truly different from a standard hematite.

The lattice parameters, for the phase with a hematite type structure inassay 7 are a_(o)=5.00821(0.001166)Å and c_(o)=13.69433(0.002883)Å.Comparison of these lattice constants to those of the hematite fromNIST, which have been experimentally determined, indicates that they aredifferent. The differences are well beyond the experimental error range.This difference may be due to impurity atoms sitting on crystallographicsites or in interstitial sites. At present, the type or position ofthese impurity atoms cannot be determined.

In a preferred embodiment, the present invention relates to a method forremoving NO_(x) contaminants from a SOx containing gaseous streamcomprising the steps of:

(1) providing a catalyst composite;

(2) in a sorbing period, passing a lean gaseous stream comprising NO_(x)and SO_(x) within a sorbing temperature range through the catalystcomposite to sorb at least some of the NO_(x) contaminants and therebyprovide a NO_(x) depleted gaseous stream exiting the catalyst compositeand to sorb and abate at least some of the SO_(x) contaminants in thegaseous stream and thereby provide a SO_(x) depleted gaseous streamexiting the catalyst composite;

(3) in a NO_(x) desorbing and abating period, changing the lean gaseousstream to a rich gaseous stream to thereby reduce and desorb at leastsome of the NO_(x) contaminants from the catalyst composite and therebyprovide a reduced NO_(x) enriched gaseous stream exiting the catalystcomposite;

(4) in a SOx desorbing period, changing the lean gaseous stream to arich gaseous stream and raising the temperature of the gaseous stream towithin a desorbing temperature range to thereby reduce and desorb atleast some of the SO_(x) contaminants from the catalyst composite andthereby regenerate the catalyst composite and provide a reduced SO_(x)enriched gaseous stream exiting the catalyst composite; and wherein thecatalyst composite comprises:

(a) a platinum component;

(b) a support; and

(c) a NOx sorbent component comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of the constituents ofthe NOx sorbent component;

(ii) subjecting the suspension or solution of the constituents of theNOx sorbent component to a temperature from about 150° C. to about 300°C. in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

In another preferred embodiment, the present invention relates to acatalyst composite prepared by a hydrothermal synthesis method whichcomprises the steps of:

(a) providing an aqueous suspension or solution of a NOx sorbentcomponent comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

(b) subjecting the suspension or solution of the constituents of the NOxsorbent component to a temperature from about 150° C. to about 300° C.in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(c) separating the precipitate from the mother liquor; and

(d) forming an admixture of the precipitate, a platinum component, and asupport.

In yet another preferred embodiment, the present invention relates to amethod of forming a catalyst composite which comprises the steps of:

(a) providing an aqueous suspension or solution of a NOx sorbentcomponent comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

(b) subjecting the suspension or solution of the constituents of the NOxsorbent component to a temperature from about 150° C. to about 300° C.in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(c) separating the precipitate from the mother liquor; and

(d) forming an admixture of the precipitate, a platinum component, and asupport.

In still another preferred embodiment, the present invention relates toa method of forming a catalyst composite which comprises the steps of:

(1) forming an admixture of:

(a) a support; and

(b) a NOx sorbent component comprising the following constituents:

(i) Na₂O in an amount up to about 0.1%;

(ii) MgO in an amount up to about 1%;

(iii) Fe₂O₃ in an amount from about 10% to about 30%;

(iv) SrO in an amount from about 0.5% to about 15%;

(v) Y₂O₃ in an amount up to about 5%; and

(vi) the remainder of the NOx sorbent component being Al₂O₃;

(2) combining a water-soluble or dispersible platinum component and theadmixture from step (1) with an aqueous liquid to form a solution ordispersion which is sufficiently dry to absorb essentially all of theliquid;

(3) forming a layer of the solution or dispersion on a substrate; and

(4) converting the platinum component in the resulting layer to awater-insoluble form;

wherein the NOx sorbent component is prepared by a hydrothermalsynthesis process comprising the steps of:

(i) providing an aqueous suspension or solution of the constituents ofthe NOx sorbent component;

(ii) subjecting the suspension or solution of the constituents of theNOx sorbent component to a temperature from about 150° C. to about 300°C. in an autoclave under pressure for a time sufficient to produce aprecipitate having an average particle size from about 0.001 to about0.2 micron in a mother liquor; and

(iii) separating the precipitate from the mother liquor.

In this preferred embodiment, the NOx sorbent component may furthercomprise BaO in an amount from about 0.5% to about 15%.

The following examples are provided to further illustrate variousembodiments of this invention and to provide a comparison between theenumerated catalysts of this invention and prior art catalysts. Theexamples are provided to illustrate the nature of the claimed processand are not intended to limit the scope of the claimed invention. Unlessotherwise stated, parts and percentages in the examples are given byweight.

EXAMPLES Preparation of Sulfur Tolerant NOx Trap Materials Example 1(E1)

Weighed the following precursors: Aluminum acetate Al—Ac (Boehmite,AlOOH, containing 65.4 wt % Al₂O₃) 1422.02 g; Barium acetate Ba—Ac(containing BaO 60 wt %) 625 g; cerium nitrate Ce—N aqueous solution(containing CeO₂30.6 wt %) 637.2 g.

Barium acetate was first dissolved in deionized water. Cerium nitratewas also dissolved in deionized water. The solution had a pH of 7.62.Al—Ac was added into the Ba—Ac solution under constant stirring.Deionized water was added and the final slurry had a pH of 6.32. Ceriumnitrate solution was added into the slurry. Continued to mix for 1 hour,and pH dropped to 5.75. The slurry started to gel. Mixing was continuedovernight and the gel thickened like jelly. Additional deionized waterwas added to the gel to thin it and to bring the total volume to 12liters. The gel was transferred to a 5-gallon autoclave for hydrothermalsynthesis.

The autoclave was heated to 232° C., kept at 232° C. for 2 hours, thencooled down to room temperature. Soaked at room temperature for 60hours, the autoclave was heated up to 230° C. The hydrothermal processat 230° C. was continued for 48 hours, then cooled down to roomtemperature.

The product was centrifuged. The mother liquor had a pH of 4.68. Theprecipitate was washed with deionized water and centrifuged again. Theproduct was dried overnight and calcined at 550° C. for 2 hours in a boxfurnace. The final product had a BET surface area of 62 m²/g. XRF semiquantitative analysis indicated the final composite to be 84.7 wt %Al₂O₃-11.5 wt % CeO₂-2.4 wt % BaO.

Example 2 (E2)

Trap material E2 was made by hydrothermal synthesis and prepared asfollows. Weighed the following precursors: 45.04 g of aluminum acetateAl—Ac (Boehmite, AlOOH, containing 65.4 wt % Al₂O₃); 44.59 g of cobaltnitrate Co—N (containing 22.5 wt % of CoO); and 34.135 g of lanthanumnitrate La—N solution (containing 27.1 wt % La₂O₃).

Co—N was dissolved in La—N solution, deionized water was added tocompletely dissolve the Co salt. The final solution had a pH of 1.23.Al—Ac was added into 250 g of deionized water under constant stirring.The slurry had a pH of 5.29. The nitrate solution was added into theslurry under vigorous stirring. Gradually the mixed slurry started togel. At this stage, the measured pH was 4.96. Ammonia of 10%concentration was added dropwise, until pH increased to 7.60 and thecolor of the gel turned to blue-green. Transferred the gel intoautoclave. The hydrothermal synthesis was processed for 64 hours at 250°C. The product was washed and centrifuged, then dried and calcined at550° C. for 2 hours. XRF semi quantitative resulted in 79 wt % Al₂O₃-19wt % Co₃O₄-1 wt % La₂O₃.

Example 3 (E3)

The sample was made by hydrothermal synthesis. The composition of thesample was (XRF) 25.2 wt % BaO-11.0 wt % CeO₂-63.8 wt % Al₂O₃.

Example 4 (E4)

The sample was made by hydrothermal synthesis. The nominal compositionof the sample was 25 wt % SrO-32 wt % TiO₂-43 wt % Al₂O₃.

Example 5 (E5)

This is a sample whose composition is the same as sample Example 3 butnot made by hydrothermal synthesis. Example 5 was made byco-precipitation. Precursors cerium nitrate (39.63 wt % CeO₂), bariumacetate (60.6 wt % BaO), aluminum acetate (65.5 wt % Al₂O₃), were used.Al—Ac was added into the Ba—Ac aqueous solution under vigorous stirringto make a slurry. Acetic acid was added dropwise into the slurry. pHdecreased to between 6 and 5, and the slurry started to gel. Ce—Nsolution was added into the gel under stirring. Acetic acid was addedand pH approached 5. The gel was transferred into a crucible and driedat 150° C. overnight. The dried material was ground and sieved to 250um. The granules were calcined at 300C for 4 hours then 450° C. for 6hours.

Preparation of the Trap Testing Samples for Mini-Reactor Examples of NOxTrap Testing Samples (Samples T1 to T5)

Sample preparation: The above materials (E1 to E5) were impregnated withanionic Pt solution to obtain 1 wt % of Pt loading dispersed on the trapmaterial. The impregnated sample was dried and calcined at 450° C. for 1hour to form dry chunks. The chunks were crushed and sieved to obtainparticles of diameter of average 300 um grains. Thus, corresponding tothe trap materials E1 to E5, trapping test samples were obtained anddesignated as T1 to T5 what were 300 um granular particles loaded with 1wt % Pt.

Test Methods of NOx Trapping and Sulfur Regeneration Testing Protocoland CO Injection Mode

Lean gas flow was used to load the trap with NOx and SOx; rich flow wasused to release NOx (and SOx) and regenerate sulfur from the sample.Synchronized valves were used to simultaneously close/open O2/CO gaslines to inject CO (or vise versa to inject O2).

O₂ NO CO N₂ SO₂ Rich  0 500 ppm 2.2% balance 60 ppm Lean 10% 500 ppm 0balance 60 ppm

The Lean/Rich cycle was set alternatively at 5 minutes lean and 5seconds rich with VHSV 50,000/hr. or 25,000/hr.

Note that a high level of SOx (60 ppm) was used in this test. Moderncommercial diesel fuel containing 350 ppm sulfur would generate 20 ppmSO2 in the exhaust gas stream. Hence, the 60 ppm SOx level used in thistest is 3 times that expected in modern commercial diesel fuel.

NOx and SOx base lines were determined first with an empty tube prior tothe test. Sample of the trap material was loaded in the reactor tubewith a 50:50 mixture with cordierite particles. In an alternativelean/rich mode, NO level, as a function of sulfur poisoning andregeneration, was observed and recorded.

After the experiment started, the NO level in the exit of the catalystincreases gradually with time, because more and more sulfur was adsorbedon the NOx trap. The degradation of NO conversion is an indication of“the degree of NOx trap being poisoned in SO2 environment.” A degradedNOx trap was then regenerated at a given temperature. Observation andrecording of NOx trap performance was then repeated. A good regeneratedNOx trap should show a NO conversion level as if it had not beenpoisoned.

Unless specified, the NOx performance test was carried out at a constanttemperature, 300° C.

Test Procedure

Sample is aged at 700° C. for 2 hours in air (oven) before test.

NOx and SOx base lines were determined without catalyst first in thereactor prior to run the sample.

Test tube loading: sample T2 was loaded in the reactor tube as wellmixed (100 mg trap material)+(100 mg cordierite, diluent); other samples(50 mg trap material)+(50 mg cordierite); total gas flow was 100 sccm.There was a VHSV: 25,000/hr and an equivalent of 200 g/(ft)3 Pt loadingfor T2; VHSV: 50,000/hr and an equivalent of 100 g/(ft)3 Pt loading forother samples (T1, T3, T4 and T5).

At 300° C. constant temperatures, ran lean/rich cycles for 2 hours andrecorded the level of NO changes. When the catalyst was poisoned by S,the NOx level should increase gradually. Then the NOx/SOx trap wasregenerated at a given temperature (e.g. 450° C., 500° C., 550° C.) for15 minutes. After regeneration of S, NOx trap function was retested.Compare the NOx level with original level to verify the effectiveness ofregeneration.

Result 1 Comparison of Performance of NOx Trap Samples in Lean/Rich ModeBeing Sulfated and Regenerated

The % NO conversion is tabulated in the following table. Higher % NOconversion indicates a more effective NOx trap performance, lower % NO aless effective NOx trap, either poisoned by sulfur and/or lessregenerated by the regeneration process.

% NO conversion—poisoned by SO2 and the recovery after regeneration

sample A A′ B C D regeneration T1 (E1) 68% — 46% — 68% 400° C. T2 (E2)*75% — 74% 73% 77% 400° C. T3 (E3) 58% — 46% — 70% 550° C. T4 (E4) 77% —68% — 77% 500° C. T5 (E5) 31% 19% — — — **cannot be regenerated @ 550 C.*100 mg loading (25,000 VHSV) **T5 had the same composition as T3 butwas not synthesized by hydrothermal process. T5 was made byco-precipitation method. T5 had a NOx conversion activity much lowerthan T3. In addition, it was found that T5 could not be regenerated at550° C. A: initial NO conversion of a fresh sample w/o sulfur A′:exposed to 60 ppm SO₂ for 15 minutes B: exposed to 60 ppm SO₂ for 1hour. C: exposed to 60 ppm SO₂ for 2 hours. D: after regeneration for 15minutes.

Results 2 Performance of Continuous Trapping NOx in Prolong Length ofTime in Sulfur Environment Without Regeneration

Sample T2 has exposed to 60 ppm SO₂ for prolong length of time. After 22hours, the NO conversion level kept at 49%. At 300° C. workingtemperature, sulfur could be partially regenerated and therefore the NOxtrap T2 remained active (at a lower level). In this sense, T2 was asulfur resistant NOx trap.

Example 6 (E6)

This example illustrates a NOx trap sample prepared with Fe as one ofthe key components. The results of a model gas reactor clearly showedthat the sample was capable of partially regenerate sulfur at the 300°C. operation temperature, while it also performed NOx trap function atthe same time. The NOx trap was a sulfur resistant NOx trap and it wouldmaintain certain amount of NOx activity for practically no limit oftime.

Forty five point zero four grams of aluminum acetate Al—Ac (65.5 wt %Al₂O₃), 55.80 grams of iron nitrate Fe—N (14.0 wt %, Fe), and 34.14grams of lanthanum nitrate La—N solution (27.1 wt % La₂O₃) precursorswere weighed. Iron nitrate was dissolved in lanthanum nitrate solution.A slurry of Al—Ac was made with 200 grams of D. I. Water. The slurry wasconstantly stirred for 2 hours and started to gel. The nitrate solutionwas added into Al—Ac slurry. A thick dark brown gel was formed. The gelwas aged over night. A pH 3.5 was measured. The aged gel was submittedfor hydrothermal treatment at 250° C. for 72 hours. The precipitate wasseparated from mother liquor, washed once, and dried over night. Themother liquor had a pH<3.

Preparation of the Test Sample for Mini-Reactor Example T6

The same test sample preparation method was used in preparing T6. T6 wasmade from E6, having granular shape of 300 um diameter and loaded with 1wt % Pt.

Testing Results

Test method was the same as used in T1 to T5, with a 50 mg sampleloading and 50,000/hr. VHSV. In 60 ppm SO₂ environment, the time zero NOconversion was 57%, then reduced to 47% after 1 hour, and 50% 2 hours.However, the zero time activity increased to 70% after a 500Cregeneration. After a long period of continuous test, the trap stillshowed NOx activity. After 68 hours, the NO conversion was 47%.Considering that the amount of SO₂ flew through was (calculated) about 1mg per hour, there were 68 mg of SO₂ passing the 50 mg sample in 68hours. This clearly showed that the sample was capable of partiallyregenerate sulfur at 300° C. operation temperature, while it alsoperformed NOx trap function at the same time. NOx trap T6 was a sulfurresistant NOx trap and it would maintain certain amount of NOx activityfor practically no limit of time. The sulfur regeneration temperaturewas quite low. After the 68 hours continuous test, the NOx activity wasrecovered with 400° C. sulfur regeneration.

Example 7

Assay 7 was hand ground in a mortar and pestle. White Rock 10μ quartzwas added as an internal standard and the two powders were homogenized.The mixture was then backpacked into a cavity in a flat plate mount.X-ray diffraction data was collected with a Philips vertical goniometerwith generator settings of 45 kV and 40 mA. The scan range was from 20°to 900°2θ using a step size of 0.02°2θ and a 10 second count time perstep. The assay contains bohmite, a transition alumina, and a phaseclose in structure to hematite. Quartz, the internal standard is alsopresent, see the spectra below. The peaks corresponding to those of thequartz internal standard were profile fit first to determine the peakcentroid and intensity. A calibration curve was generated by apolynomial fit to this data. The raw data was then calibrated using thiscurve. Next the peaks corresponding to those of the “hematite” phasewere profile fit to determine their peak centroids and intensities. Aunit cell was refined using the profile fit peak locations, the hklindices of each peak, and the space group for hematite which is R-3c.Initial lattice parameters were taken from reference card 33-664. Tocheck this work data was collected on a NIST iron ore standard composedprimarily of hematite. Traces of quartz and maghemite were also present.The assay number for this sample is D20328N. The scan range was from 19°to 86°2θ using a step size of 0.02°2θ and a 10 second count time perstep. White Rock 10μ quartz was used as the internal standard. The rawdata was processed in the same fashion as assay 7.

While the invention has been described in detail with respect tospecific embodiments thereof, such embodiments are illustrative and thescope of the invention is defined in the appended claims.

We claim:
 1. A method for removing NOx contaminants from a SOxcontaining gaseous stream comprising the steps of: (1) providing acatalyst composite; (2) in a sorbing period, passing a lean gaseousstream comprising NOx and SOx within a sorbing temperature range throughthe catalyst composite to sorb at least some of the NOx contaminants andthereby provide a NOx depleted gaseous stream exiting the catalystcomposite and to sorb and abate at least some of the SOx contaminants inthe gaseous stream and thereby provide a SOx depleted gaseous streamexiting the catalyst composite; (3) in a NOx desorbing and abatingperiod, changing the lean gaseous stream to a rich gaseous stream tothereby reduce and desorb at least some of the NOx contaminants from thecatalyst composite and thereby provide a reduced NOx enriched gaseousstream exiting the catalyst composite; (4) in a SOx desorbing period,changing the lean gaseous stream to a rich gaseous stream and raisingthe temperature of the gaseous stream to within a desorbing temperaturerange to thereby reduce and desorb at least some of the SOx contaminantsfrom the catalyst composite and thereby regenerate the catalystcomposite and provide a reduced SOx enriched gaseous stream exiting thecatalyst composite; and wherein the catalyst composite comprises: (a) aplatinum component; (b) a support; and (c) a NOx sorbent componentcomprising the following constituents: (i) Na2O in an amount up to about0.1%; (ii) MgO in an amount up to about 1%; (iii) Fe2O3 in an amountfrom about 10% to about 30%; (iv) SrO in an amount from about 0.5% toabout 15%; (v) Y2O3 in an amount up to about 5%; and (vi) the remainderof the NOx sorbent component being Al2O3; wherein the NOx sorbentcomponent is prepared by a hydrothermal synthesis process comprising thesteps of: (i) providing an aqueous suspension or solution of theconstituents of the NOx sorbent component; (ii) subjecting thesuspension or solution of the constituents of the NOx sorbent componentto a temperature from about 150° C. to about 300° C. in an autoclaveunder pressure for a time sufficient to produce a precipitate having anaverage particle size from about 0.001 to about 0.2 micron in a motherliquor; and (iii) separating the precipitate from the mother liquor. 2.The method according to claim 1, wherein the NOx sorbent componentfurther comprises BaO in an amount from about 0.5% to about 15%.
 3. Themethod according to claim 1, wherein the SOx desorbing temperature rangein (4) is greater than about 300° C.
 4. The method according to claim 3,wherein the SOx desorbing temperature range in (4) is greater than about350° C.
 5. The method according to claim 4, wherein the SOx desorbingtemperature range in (4) is greater than about 400° C.
 6. The methodaccording to claim 5, wherein the SOx desorbing temperature range in (4)is greater than about 450° C.
 7. The method according to claim 1,further comprising a platinum group metal component other than platinum.8. The method according to claim 7, wherein the platinum group metalcomponent is selected from the group consisting of palladium, rhodium,ruthenium, and iridium components, and mixtures thereof.
 9. The methodaccording to claim 1, wherein the support is selected from the groupconsisting of alumina, silica, titania, and zirconia compounds.
 10. Themethod according to claim 1, wherein the support is selected from thegroup consisting of activated alumina, alumina-ceria, alumina-chromia,alumina-silica, alumina-zirconia, silica, silica-titania,silica-titania-alumina, silica-titania-zirconia, titania, zirconia,zirconia-titania, and zirconia-alumina-titania.
 11. The method accordingto claim 1, wherein the catalyst composite comprises: (i) at least about19/ft3 of the platinum component; (ii) from about 0.15 g/in3 to about6.0 g/in3 of the support; and (iii) from about 0.025 g/in3 to about 4g/in3 of the NOx sorbent component.
 12. The method according to claim 1,wherein the precursors of the first metal oxide and second metal oxideare water-soluble/dispersible metal salts selected from the groupconsisting of acetates, nitrates, hydroxides, oxychlorides,hydroxychlorides, carbonates, sulfates, oxalates, and tartrates.
 13. Themethod according to claim 1, wherein the suspension or solution of metalhydroxides in (4)(c)(ii) is subjected to a temperature from about 175°C. to about 250° C.
 14. The method according to claim 1, wherein theprecipitate in (4)(c)(ii) has an average particle size from about 0.01to about 0.2 micron.
 15. The method according to claim 1, wherein thecatalyst composite is supported on a metal or ceramic honeycomb carrieror is self-compressed.